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Nondestructive Formation of Supramolecular Nanohybrids of Single-Walled Carbon Nanotubes with Flexible Porphyrinic Polypeptides Kenji Saito,† Vincent Troiani,‡ Hongjin Qiu,‡ Nathalie Solladie´ ,*,‡ Takao Sakata,§ Hirotaro Mori,§ Mitsuo Ohama,|| and Shunichi Fukuzumi*,† Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, SORST, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan, Groupe de Synthe` se de Syste` mes Porphyriniques (G2SP) Laboratoire de Chimie de Coordination du CNRS, UPR 8241, 205 route de Narbonne, 31077 Toulouse Cedex 4, France, Research Center for Ultra High Voltage Electron Microscopy, Osaka UniVersity, 7-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan, and Department of Chemistry, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 565-0043, Japan ReceiVed: August 29, 2006; In Final Form: October 22, 2006
Strategies for single-walled carbon nanotube (SWNT) separation are critical to developing nanotubes as useful nanoscale building blocks. Herein, diameter-selective dispersion of HiPco single-walled carbon nanotubes has been accomplished through noncovalent complexation of the nondestructive nanotubes with a flexible porphyrinic polypeptide bearing 16 porphyrin units [P(H2P)16] in DMF at 298 K. Supramolecular formation occurs through wrapping of peptidic backbone in P(H2P)16 and π-π interaction between porphyrins and nanotubes to extract the large-diameter nanotubes (ca. 1.3 nm) as revealed by ultraviolet-visible-near-infrared and Raman spectroscopy as well as high-resolution transmission electron micropscopy. The photoexcitation of the supramolecular complex affords the long-lived charge-separated state.
Introduction The novel and unique electronic properties of carbon nanotubes, in particular single-walled carbon nanotubes (SWNTs), make them excellent candidates for next-generation nanoelectronic applications.1-3 However, current preparative methods for SWNTs generate heterogeneous nanotube mixtures that can vary in length, diameter, and electronic properties (semiconducting, semimetallic, and metallic) and prevent a wide range of fundamental researches and applications. Because unmodified SWNTs are significantly hydrophobic and readily aggregate, sample homogeneity is one key issue. Extensive efforts have so far been devoted to control the length, diameter, and electronic properties of SWNTs.4-19 In most cases, however, the oxidative treatment and vigorous sonication of SWNTs involved in the separation treatment are required, resulting in shortened nanotubes and randomly breaks the carbon shell to produce a high degree of degradation.20,21 Thus, the nondestructive extraction according to length and/or diameter from raw SWNTs is highly desired for a promising route toward a variety of applications.22-24 In this context, selective interaction of large extended π-electron systems with SWNTs have merited special attention.25-27 We report herein our discovery that porphyrin-peptide hexadecamer [P(H2P)16 in Figure 1] nondestructively forms supramolecular nanohybrids with HiPco SWNTs. In addition, selective interaction of P(H2P)16 with SWNTs by utilizing noncovalent interaction such as wrapping of peptide backbone in P(H2P)16 and π-π interaction between porphyrins and * Corresponding authors. E-mail:
[email protected];
[email protected]. † Osaka University, SORST, and JST. ‡ Laboratoire de Chimie de Coordination du CNRS. § Research Center for Ultrahigh Voltage Electron Microscopy. | Osaka University.
nanotubes results in diameter-selective separation of SWNTs from starting purified SWNTs. This approach allows us not only to achieve nondestructive and diameter-selective SWNT extraction but also to develop an efficient light-energy conversion system.22-24 Experimental Methods The synthetic procedure of porphyrin-peptide hexadecamer P(H2P)16 is as follows. It is based on the oligomerization of a porphyrin functionalized enantiopure L-lysine derivative bearing a free-base chromophore. The synthesis of the L-lysine derivative functionalized by a porphyrin as well as the elaboration of the dipeptide, the tetrapeptide, and the octapeptide were carried out as described previously.28 The BOC protective group of P(H2P)8 was cleaved with 71% yield by stirring the substrate for 7 h at room temperature in a 0.7 M solution in dichloromethane of trimethylsilylchloride and phenol. Alternatively, the carboxylic acid function of P(H2P)8 was deprotected with 52% yield in a 4:1 mixture of dimethylacetamide/piperidine in the presence of 1.0 equiv of tetrakistriphenylphosphine palladium(0) at room temperature for 28 h. A peptidic coupling reaction between P(H2P)8-NH2 and P(H2P)8-COOH was carried out in dichloromethane in the presence of 1.5 equiv of N,N′-dicyclohexylcarbodiimide and 1.5 equiv of 1-hydroxybenzotriazole to produce the hexadecamer P(H2P)16. After 42 h at room temperature and tedious column chromatographies, the hexadecamer was isolated with 37% yield. P(H2P)16 has been characterized by MALDI-TOF mass spectrometry (m/z ) 187 77.81; [M + H]+ calcd, 187 77.54). Further metalation of P(H2P)16 with Zn(II) acetate dihydrate in a 5:1 mixture of chloroform/MeOH under reflux for 10 h gave P(ZnP)16 with 87% yield. SWNTs were purchased from Carbon Nanotechnologies, Inc., Houston, TX (HiPco). It was purified according to the reported
10.1021/jp065615i CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006
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Figure 1. Structure of porphyrin-peptide hexadecamer [P(H2P)16].
Figure 2. (a) UV-vis-NIR absorption spectra of P(H2P)16/SWNTs (blue) and P(H2P)16 (black) in DMF. (b) Comparison of the absorption between P(H2P)16/SWNTs (blue) and SWNTs suspension (black) in DMF.
procedure to obtain purified SWNTs.29 The general procedure to construct the supramolecular nanohybrid composed of P(H2P)16 and SWNTs is as follows: A mixture of P(H2P)16 (1.58 mg, 4.2 × 10-5 M) and purified SWNTs (0.5 mg) were suspended in 2 mL of DMF by mild sonication (i.e., 70 W for 15 min). The mixture was heated at 100 °C for 10 h, followed by stirring overnight at room temperature. The heating and stirring cycle were repeated at 100 °C for 24 h. After centrifugation for 30 min at 17 900g, the centrifuged solid was resuspended in DMF by sonication (70 W for 15 min). Unreacted SWNTs were separated by 30 min centrifugation at 7940g to obtain a dark transparent supernatant solution. Ultraviolet-visible-near-infrared (UV-vis-NIR) absorption spectrum was measured on a Shimadzu UV-3100PC spectrometer. The fluorescence lifetimes were measured by a singlephoton counting method using a second harmonic generation (SHG, 410nm) of Ti:sapphire laser (Spectra-Physics, Tsunami 3950-L2S, fwhm 1.5 ps) and a streakscope (Hamamatsu Photonics, C4334-01) equipped with a polychromator (Action Research, SpectraPro 150) as an excitation source and a detector, respectively. For nanosecond laser flash photolysis experiments, deaerated DMF suspension of supramolecular nanohybrids were excited by an Nd:YAG laser (Continuum, SLII-10, 4-6 ns fwhm) at λ ) 440 and 427 nm with the power of 7, 3, and 1 mJ per pulse. The transient spectra were recorded using fresh solutions in each laser excitation. The solution was deoxygenated by argon purging for 10 min prior to measurements. Highresolution transmission electron microscopy (HRTEM) analyses
were performed under a HITACHI H-9000 operating at an accelerating voltage of 300 kV. Raman Spectra were measured with a JASCO NR-1800 spectrometer using laser excitation at 514.5 and 632.8 nm. Results and Discussion Supramolecular Formation between P(H2P)16 and SWNTs. The progress of supramolecular formation between porphyrins of P(H2P)16 and nanotubes was easily recognized by the color change of the solution in the mixture. The color of the supernatant solution drastically changes from dark red to pale during the reaction. This suggests that P(H2P)16 molecules are adsorbed on precipitated SWNTs. In contrast, no color change leading to homogeneous suspension was observed in the case of P(H2P)8.28,30 Judging from no difference in the length of the chromophore linkers, supramolecular formation was attained through wrapping of polypeptidic chain in P(H2P)16 around the circumference of the SWNTs.15 Figure 2a shows the ultraviolet-visible-near-infrared (UVvis-NIR) absorption spectrum of supramolecular nanohybrids in DMF. The Soret band of porphyrins in P(H2P)16 is 10 nm red-shifted as compared with that of P(H2P)16 (Figure 2a).31 This clearly indicates the occurrence of π-π interaction between porphyrins and SWNTs. Because no supramolecular formation was observed in the case of P(H2P)8 as described above, the long polypeptidic chain in P(H2P)16 plays an important role in facilitating π-π interaction between porphyrins and SWNTs.
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Figure 3. HRTEM images of P(H2P)16/SWNTs (scale bar ) 20 nm); (a) an individual nanotube and (b) individual nanotubes with small-diameter bundles.
Figure 4. Raman Spectra for pristine SWNTs (film, black) and SWNTs after removal of P(H2P)16 (film, blue) at 514.5 nm excitation. (a) RBMs. RBMs were normalized to the band at ∼200 cm-1. (b) D and G band regions. The obtained spectra are normalized to the ωG+ feature.
We also examined the comparison of the absorption between P(H2P)16/SWNTs and SWNTs suspension in DMF as shown in Figure 2b. The absorption in the first interband transition for semiconductor (S11: 900-1600 nm) of supramolecular nanohybrids is sharpened as compared with that of pristine SWNTs. This may arise from the enrichment of individual SWNTs in the P(H2P)16/SWNTs suspension because large bundles of nanotubes (i.e., poor dispersions) exhibit monotonically decreasing absorbance with increasing wavelength.32 This results from wrapping of the polypeptidic chain in P(H2P)16 around the circumference of the SWNTs. Supramolecular formation between P(H2P)16 and SWNTs was also checked by using high-resolution transmission electron microscopy (HRTEM). It has been confirmed that no largediameter bundles are observed for the supramolecular complex between P(H2P)16 and SWNTs. Instead, an individual nanotube of a diameter below 2 nm and individual nanotubes with smalldiameter bundles are observed as shown in Figure 3 (part a and b, respectively). These results are in good agreement with that of the UV-vis-NIR spectra. Such successful debundling of SWNTs may be attained because of the supramolecular formation with P(H2P)16. To determine the diameters of the SWNTs wrapped with porphyrinic polypeptide, we investigated the Raman spectra
from the drop-cast film after the removal of adsorbed P(H2P)16 in comparison of those of pristine SWNTs (Figure 4 and 5). The removal of adsorbed P(H2P)16 from the nanohybrid sample was accomplished by washing the sample with excess DMF and THF, followed by filtration. The washing and filtration cycle were repeated several times to obtain P(H2P)16-free SWNTs. This procedure is necessary to prevent the excitation of porphyrin Q-band in the supramolecular nanohybrids, allowing one to examine the Raman spectroscopy with virtually the same aggregation state.33 Three characteristic features have been identified in the Raman spectra of SWNTs: a radial breathing mode (RBM) in the frequency range of 100-200 cm-1, a disordered carbon mode (D band) around 1300 cm-1, and the G band around 1600 cm-1 originating from the tangential oscillation of the carbon atoms in the SWNTs. All Raman lines obtained were treated with a linear baseline subtraction and normalized with respect to the band at ∼200 cm-1. In general, normalization at a specific RBM feature allows us to evaluate the relative intensities of different nanotube species present in the pristine and P(H2P)16-treated nanotube samples, respectively. Figure 4 shows Raman spectra with an excitation energy of 514.5 nm, which is resonant with v1 f c1 transitions in relatively small-diameter metallic nanotubes and v3 f c3 transitions in large-diameter semiconducting nanotubes.16,34-36
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Figure 5. Raman spectra for pristine SWNTs (film, black) and SWNTs after removal of P(H2P)16 (film, blue) at 632.8 nm excitation. (a) RBMs normalized to the band at ∼200 cm-1. (b) D and G band regions. The obtained spectra are normalized to the ωG+ feature.
For the pristine SWNTs sample, five distinct RBM peaks are observed near 189, 209, 250, 264, and 271 cm-1, corresponding to SWNTs with approximate diameters of 1.27, 1.14, 0.94, 0.89, and 0.86 nm, using the equation d (nm) ) 223.5/(ωRBM (cm-1) - 12.5) as the correlation between diameter and RBM frequencies (Figure 4a).37 The RBM bands near 209, 250, 264, and 271 cm-1, attributable to small-diameter nanotubes, in SWNTs after removal of P(H2P)16 exhibit significantly diminished intensity as compared with that of the band near 189 cm-1, which is a selective enhancement of large-diameter semiconducting SWNTs in the sample. The pristine SWNTs had two metallic components with a Breit-Wigner Fano (BWF) line and four semiconducting components with Lorenzian lines in the G band region. The intensity of semiconducting components of SWNTs after removal of P(H2P)16 is slightly reduced as compared to the pristine SWNTs as shown in Figure 4b.38 We also observed the Raman spectra with an excitation energy of 632.8 nm excitation. Smaller diameter semiconducting tubes with resonant v2 f c2 transitions and larger diameter metallic nanotubes with resonant v1 f c1 transitions are seen in Figure 5a.35,39 The band near 197 cm-1 (∼1.21 nm diameter SWNTs) is significantly enhanced in intensity as compared with that of the bands near 221, 258, and 284 cm-1 (corresponding to SWNTs with approximate diameter between 0.82 and 1.07). These observations suggest that P(H2P)16 preferentially interacts with large-diameter SWNTs irrespective of the metallicity of SWNTs. The selective interaction of P(H2P)16 with a largediameter nanotube may result from the appropriate orientation of P(H2P)16 that can wrap the large-diameter SWNTs through π-π interaction. The intensity of semiconducting components of SWNTs after removal of P(H2P)16 in the G band region is also slightly reduced as compared to the pristine SWNTs in Figure 5b.38 The π-π interaction between aromatic molecules and SWNTs has been reported to be dependent on the electronic structure of SWNTs because the molecular size is large enough and the effective contact area and the atomic correlation are simultaneously optimized on metallic SWNTs.25 In the case of P(H2P)16, the interaction with metallic SWNTs is slightly favored as compared to that with semiconducting SWNTs. Moreover, it is well-known
Figure 6. Fluorescence time profiles of P(H2P)16/SWNTs (blue line) and P(H2P)16 (black line) in DMF at 298 K.
that the D band is the diagnostic of disruptions in the hexagonal framework of the SWNTs. As shown in Figures 4b and 5b, no distinctive difference was found. Thus, we conclude that our functionalization process causes little or no damage to the structure of SWNTs. Photodynamics of the Supramolecular Complex. The photodynamics of the supramolecular complex [P(H2P)16/ SWNTs] was examined by time-resolved fluorescence measurements. The fluorescence time profiles of P(H2P)16/SWNTs and P(H2P)16 are shown in Figure 6. The fluorescence lifetime of P(H2P)16/SWNTs in DMF is determined to be 6.9 ns, which is slightly shorter than that of P(H2P)16 (7.6 ns). Laser photoexcitation of P(H2P)16/SWNTs in DMF results in the appearance of the transient absorption with bleaching of the porphyrin bands (518, 553, 597, and 649 nm) and SWNTs (750 nm) as shown in Figure 7a (closed circles). The prominent transient absorption band appears at 450 nm together with a broad absorption band in the 500-800 nm region, which exhibits a number of bleaching bands. The bleaching bands agree with the absorption bands due to P(H2P)16/SWNTs (see the comparison with the solid line absorption spectrum in Figure 7a). Such agreement indicates that the observed spectrum is well resolved and the intensity is higher than the noise level. By
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Figure 7. (a) Transient absorption spectra of P(H2P)16/SWNTs (solid line with black circles) and P(H2P)16 (solid line with white circles) in DMF at 298 K taken at 6 µs after laser excitation at 440 and 427 nm, respectively. Inset: UV-vis-NIR absorption spectra of P(H2P)16/SWNTs. (b) First-order plots at 480 nm with different laser powers (7, 3, and 1 mJ, respectively).
taking into account the bleaching bands, the observed transient absorption spectrum agrees with that of the free base porphyrin radical cation.40 In contrast, no transient absorption is observed upon photoexcitation of P(H2P)16 (open circles in Figure 7a). Thus, the observed transient absorption spectrum is assigned to the charge-separated state of P(H2P)16/SWNTs that is produced by photoinduced electron transfer from the singlet and triplet excited states of P(H2P)16 to SWNTs. The triplet excited state may play a major role in the photoinduced electron transfer because the singlet lifetime is only slightly reduced in P(H2P)16/ SWNTs (Figure 6). The decay of the absorbance at 480 nm obeys the first-order kinetics with the same slope irrespective of the difference in the laser intensities as shown in Figure 7b. If the decay of the porphyrin radical cation results from the bimolecular reaction due to disproportionation,41 then the decay kinetics would be second-order, when the decay profile would be highly dependent on the initial concentration of the porphyrin radical cation produced with different laser intensities. Thus, the same slope irrespective of difference in the laser intensities in Figure 7b strongly indicates that the decay of the porphyrin radical cation results from the back electron transfer from the reduced SWNTs to the porphyrin radical cation in the supramolecular complex. The lifetime of the transient species is determined from the slope as 0.37 ( 0.03 ms, which is significantly shorter than the reported triplet lifetimes of porphyrin peptide oligomers.30 In conclusion, P(H2P)16 forms a supramolecular complex preferentially with large-diameter SWNTs by wrapping of peptidic backbone in P(H2P)16 and π-π interaction between porphyrins and nanotubes, leading to debundling of nanotubes and diameter-selective separation of SWNTs from starting purified SWNTs. The long-lived charge-separated state is attained upon photoexcitation of the supramolecular nanohybrids. This method opens a new strategy toward the extraction of large-diameter SWNTs without destruction of the π-conjugated system within SWNTs and the development of efficient light energy conversion. Acknowledgment. This work was partially supported by a Grant-in-Aid (No. 16205020) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We thank Dr. Yasuyuki Araki and Prof. Osamu Ito for fluorescence lifetime measurements. K.S. expresses his special thanks for the center
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